Conventional methods consist of taking a certain number of views of the object by a two-dimensional network of sensors which is successively installed with diverse orientations around the object. The sensors measure the sums or projections of a function or property to be measured on lines traversing the object. The lines may be rays emitted by a source whose total attenuation is measured through the object, or a composite ray produced by the overall transmission activity of the points of the object situated on the focussing lines of each sensor. These two situations scarcely differ as regards the reconstructing and calculation principles as everything occurs in one second if the ray were emitted from a visual source. The combination of the attentuation and emission sums makes it possible to refine the value taken by the corresponding property of the object at each point of a rendering discrete network.
In order to reconstruct the image, it is possible to use analytic methods in which the measurements undergo an analytic transformation, such as a Fourier or Radon transformation, followed by a formula for inverting the transform of the measurements applied before obtaining the values of the property to the points of the network for rendering discrete drawbacks due to the number of calculations may be significantly attenuated if an item of information or the information a priori concerning the object is/are introduced. In fact, there is frequently a pre-established fairly accurate notion of the distribution of the property (in terms of density, shape or texture, etc) measured at least in one portion of the object, and the introduction of the corresponding description as an initial iteration stage or definitive result for certain zones of the object further improves the result and may accelerate convergence.
However, the invention concerns reconstructions conducted on changing objects, other problems then appearing as the successive measurements are obtained for different states of the object, which renders the reconstructions blurred, whether the reconstruction method is algebraic or analytical.
Several solutions have been put forward for resolve this drawback. For example, it is possible to take all the measurements at the same time, which requires that the device is equipped with the corresponding number of detecting networks so as to obtain extremely clear images, the resolution of the system being in this instance complete. This solution is less realistic as being too expensive and difficult to implement. Another solution used consists of taking solely one portion of the measurements simultaneously, which does not make it possible to obtain extremely clear images, the resolution of the system not being complete. For cyclic phenomena, such as physiological phenomena (cardiac optical image formation), it has been proposed to only take the measurements at the same point of each cycle, but all the changing phenomena of the object to be reconstructed are clearly not cyclic: this is the case with measurements where the ray is measured emitted by a radioactive tracer injected into the organism and which dilutes rapidly. There are even situations where the measurements differ significantly for cyclic phenomena. Finally, synchronized measurements on each cycle last overall for a long period and only make it possible to reconstruct images of the object in the measuring phase.